1. Field of the Invention
The present invention relates to an electrophotographic copying apparatus such as a copying machine, and more particularly, to an electrophotographic apparatus preferable for forming a picture image in a different color having an appropriate density.
2. Description of the Related Art
An electrophotographic copying apparatus provided with a developing device having a developing sleeve, a toner supply roller confronting the developing sleeve, and a toner hopper provided with the toner supply roller has been proposed.
In the conventional electrophotographic copying apparatus, toner particles (i.e., toner) in the toner hopper are supplied to the outer surface of the developing sleeve by the toner supply roller. The toner supplied to the outer surface of the developing sleeve is mixed with carriers held thereon, whereby the toner and the carrier are charged with polarities opposite to each other to form a developer consisting of two components. At this time, the mixing ratio between the toner and carrier is controlled on the basis of the difference of the bias voltage which is applied to the developing sleeve and the toner supply roller. The developer thus formed on the developing sleeve is transported to the gap between the developing sleeve and an electrostatic latent image formed on an image-holding member. Thereafter, the charged toner is electrostatically supplied to the electrostatic latent image to form a toner image.
In this type-electrophotographic copying apparatus, a picture image is developed in a different color by exchanging the developing device for a developing device containing toner particles in the different color.
A picture image can be also developed in a color by collecting toner particles held on the outer surface of the developing sleeve into the toner hopper and exchanging the toner hopper for a toner hopper containing toner particles in the different color by adjusting the bias voltage supplied, respectively, to the developing sleeve and the toner supply roller.
However, a saturation charge amount of toner particles varies with different colors. Therefore, if a picture image is developed with the bias voltage fixed, i.e., if the constant voltage is generated between the toner supply roller and the developing sleeve in developing images in a different color, it is impossible to form the image of an original document in a predetermined density on a copy sheet.
Accordingly, it is necessary in the above-mentioned apparatus to change the bias voltage to an appropriate voltage in accordance with toner particles to be used each time a toner hopper is exchanged.